Nanostructure of serpentinisation products: Importance for water transport and low-temperature alteration

Abstract

The hydration of mantle rocks occurs at mid-ocean ridges, in subduction zones and in ophiolites where it strongly modifies the properties of the oceanic lithosphere. The nanostructure of the reaction products is poorly constrained, but it is very important, as it controls fluid transport during solid volume increase, and it influences phase reactivity during further fluid/rock interaction at low temperature. We image contacts between olivine and its hydration products at the nanoscale, in a dunite collected during the Oman Drilling Project that underwent nearly isochemical serpentinisation and solid volume increase. Olivine first reacts to form a ∼100 nm thick coating composed of a brucite/serpentine mixture, suggesting isochemical serpentinisation at this scale too. Lizardite columns ∼1 μm wide replace this mixture at its margin. The columns are embedded in a brucite-rich assemblage containing a high density of nanopores that may favour fluid transport during reaction. We interpret these observations as a nanometre-scale, two-step process. Fluid pathways are formed rather than clogged by reaction products thanks to mass transfer at a scale limited to less than 100 nm. The preservation of the fluid pathways during solid volume increase explains the observed high extents of reaction. The presence of brucite in high porosity regions may explain its preferential reaction during low temperature fluid/rock interaction.